“Austempering” may be understood as a type of heat treatment for steel including initial “austenitizing,” subsequent quenching to a temperature above the “martensite” start temperature, and holding the steel at this temperature for a specified time period.
With this procedure, which may also be referred to as “isothermal conversion” of austenite into bainite, it may be possible to obtain a steel which has fully or partially been converted from an austenitic into a bainitic structure to a degree that may be controllable depending on the objective and via temperature and holding time.
For further details on austenitizing and the above-explained austempering of steel, reference is made, for example, to E.
Zimmermann, “Materials and Materials Testing”, Hermann Schrödel Verlag, 1962, pp. 38, 39, and 74-79.
Austenitization of steel, subsequent quenching and isothermal conversion of austenite into bainite above the martensite start temperature is described, for example, in U.S. Pat. No. 5,628,045.
Austempering of previously austenitized steels has previously been performed in baths referred to as hot salt baths. For this purpose, steel may be continuously held in this hot salt bath after quenching from the beginning of the bainite conversion (referred to as bainite start) to the end of the bainite conversion (referred to as bainite finish) at a constant temperature and for an appropriate, suitably selected holding time.
Furthermore, the steel to be austempered may be converted, after the elapse of a certain holding time in the hot salt bath, in a holding furnace operated using circulating air to achieve full conversion from austenite to bainite. The holding furnace used for this purpose may have the same temperature as the hot salt bath.